A Mechanochemical Reaction Cascade for Controlling Load-Strengthening of a Mechanochromic Polymer

We demonstrate an intermolecular reaction cascade to control the force which triggers crosslinking of a mechanochromic polymer of spirothiopyran (STP). Mechanochromism arises from rapid reversible force-sensitive isomerization of STP to a merocyanine, which reacts rapidly with activated C=C bonds. The concentration of such bonds, and hence the crosslinking rate, is controlled by force-dependent dissociation of a Diels–Alder adduct of anthracene and maleimide. Because the adduct requires ca. 1 nN higher force to dissociate at the same rate as that of STP isomerization, the cascade limits crosslinking to overstressed regions of the material, which are at the highest rate of material damage. Using comb polymers decreased the minimum concentration of mechanophores required to crosslinking by about 100-fold compared to previous examples of load-strengthening materials. The approach described has potential for controlling a broad range of reaction sequences triggered by mechanical load

A Mechanochemical Reaction Cascade for Controlling Load‐Strengthening of a Mechanochromic Polymer

We demonstrate an intermolecular reaction cascade to control the force which triggers crosslinking of a mechanochromic polymer of spirothiopyran (STP). Mechanochromism arises from rapid reversible force-sensitive isomerization of STP to a merocyanine, which reacts rapidly with activated C=C bonds. The concentration of such bonds, and hence the crosslinking rate, is controlled by force-dependent dissociation of a Diels–Alder adduct of anthracene and maleimide. Because the adduct requires ca. 1 nN higher force to dissociate at the same rate as that of STP isomerization, the cascade limits crosslinking to overstressed regions of the material, which are at the highest rate of material damage. Using comb polymers decreased the minimum concentration of mechanophores required to crosslinking by about 100-fold compared to previous examples of load-strengthening materials. The approach described has potential for controlling a broad range of reaction sequences triggered by mechanical load

Mechanochromism and optical remodeling of multi-network elastomers containing anthracene dimers

International audienceMulti-network elastomers are both stiff and tough by virtue of containing a pre-stretched stiff network that can rupture and dissipate energy under load. However, the rupture of this sacrificial network in all described covalent multi-network elastomers is irreversible. Herein, we describe the first example of multi-network elastomers with a reformable sacrificial network containing mechanochemically sensitive anthracene-dimer cross-links. These cross-links also make our elastomers mechanochromic, with coloration that is both persistent and reversible, because the fluorogenic moiety (anthracene dimer) is regenerated upon irradiation of the material. In proof-of-concept experiments we demonstrate the utility of incorporating anthracene dimers in the backbone of the sacrificial network for monitoring mechanochemical remodeling of multi-network elastomers under cycling mechanical load. Stretching or compressing these elastomers makes them fluorescent and irradiating them eliminates the fluorescence by regenerating anthracene dimers. Reformable mechanochromic cross-links, exemplified by anthracene dimers, hold potential for enabling detailed studies of the molecular origin of the unique mechanical properties of multi-network elastomers

Using metal-ligand interactions to access biomimetic supramolecular polymers with adaptive and superb mechanical properties

Natural Science Foundation of China [21074103]; Fundamental Research Funds for the Central Universities [2010121018]; Scientific Research Foundation for Returned ScholarsThe development of polymer materials that exhibit excellent mechanical properties and can respond to environmental stimuli is of great scientific and commercial interest. In this work, we report a series of biomimetic supramolecular polymers using a ligand macromolecule carrying multiple tridentate ligand 2,6-bis(1,2,3-triazol-4-yl)pyridine (BTP) units synthesized via CuAAC in the polymer backbone together with transition and/or lanthanide metal salts. The metal-ligand complexes phase separate from soft linker segments, acting as physical crosslinking points in the materials. The metallo-supramolecular films exhibit superb mechanical properties, i.e., high tensile strength (up to 18 MPa), large strain at break (>1000%) and exceptionally high toughness (up to 70 MPa), which are much higher than those of the ligand macromolecule and are tunable by adjusting the stoichiometric ratio of Zn2+ to Eu3+ and the stoichiometry of metal ion to ligand. The metal-ligand hard phase domains are demonstrated to be thermally stable but mechanically labile, similar to the behaviors of covalent mechanophores. The thermal stability and mechanical responsiveness are also dependent on the compositions of metal ions. The disruption of the hard phase domains and the dissociation of metal-ligand complexes under stretching are similar to the unfolding of modular domains in modular biomacromolecules and are responsible for the superb mechanical properties. In addition, the biomimetic metallo-supramolecular materials display promising responsive properties to UV irradiation and chemicals. These well designed, created and characterized robust structures will inspire further accurate tailoring of biomimetic responsive materials at the molecular level and/or nanoscale

Self-healing metallo-supramolecular polymers from a ligand macromolecule synthesized via copper-catalyzed azide-alkyne cycloaddition and thiol-ene double "click" reactions

Natural Science Foundation of China [21074103]; Fundamental Research Funds for the Central Universities [2010121018]; Scientific Research Foundation for Returned Scholars; NFFTBS [J1210014]In this study, we develop a series of new materials that can simultaneously and reversibly self-heal without external stimuli based on metallo-supramolecular interactions. Multiple tridentate 2,6-bis(1,2,3-trizaol-4-yl)-pyridine (BTP) ligand units synthesized via a copper-catalyzed azide-alkyne cycloaddition (CuAAC) "click" reaction are incorporated into the polymer backbone of a ligand macromolecule through a thiol-ene "click" reaction. 3D transient supramolecular networks are formed from the ligand macromolecule upon coordination with transition and/or lanthanide metal ions. As compared to the ligand macromolecule, the resultant supramolecular films exhibit improved mechanical properties, such as Young's modulus, strength and toughness, which can be readily tuned by the stoichiometric ratio of Zn2+ to Eu3+ to Tb3+. The supramolecular films exhibit characteristics of weakly crosslinked networks where the storage modulus G' and loss modulus G '' scaled with normalized frequency omega a(T) by the same slope of 0.5. Both the supramolecular bulk films and gels are found to exhibit fast and effective self-healing properties by virtue of the kinetically labile nature of the metal-ligand interactions

CHAPTER 1: Mechanochemistry: Inspiration from Biology

© The Royal Society of Chemistry 2018. Mechanochemistry refers to the study of the evolution of the formation and disruption of chemical bonds upon application of an external force. In this chapter, the roles of mechanical forces in different biological systems are highlighted along with mechanisms and mechanotransduction pathways showing how complex biological systems can provide inspiration for materials design. Examples of how mechano-based systems have been mimicked by other scientists are also discussed including self-healing systems

Single-molecule observation of mechanical isomerization of spirothiopyran and subsequent Click addition

peer reviewedSpirothiopyran (STP) is particularly attractive when used as a mechanophore to endow polymers with both damage-signaling and self-reinforcing capacity. It is, however, not clear the actual force required to induce the cycloreversion of STP into ring-opened thiomerocyanine (TMC), which reacts spontaneously with activated C-C bonds. Here, we used atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS) to study the mechanochemistry of STP mechanophore. It is found that the ring-opening of STP at room temperature requires forces of ∼ 200–400 pN, depending on the pulling speed. In addition, the reversibility of STP to TMC isomerization is demonstrated. Finally, mechanochemically induced intermolecular Click addition is achieved in single molecule level by pulling STP in the presence of maleimide. [Figure not available: see fulltext.

One-step functionalization of graphene with cyclopentadienyl-capped macromolecules via Diels-Alder "click" chemistry

Natural Science Foundation of China [21074103]; Fundamental Research Funds for the Central Universities [2010121018]; Natural Science Foundation of Fujian Province [2010J05033, E0820001]; Province Science Foundation of Xiamen [3502Z20101018, 3502Z20103032]Under mild conditions at ambient temperature, as well as at 80 degrees C, graphene chemically converted from graphene oxide was functionalized with cyclopentadienyl (Cp)-capped poly(ethylene glycol) monomethyl ether through a one-step Diels-Alder [4 + 2] (DA) "click" reaction without any catalyst. The resulting material showed improved dispersion properties in various solvents. Spectroscopic tools (Raman, FTIR, XPS, XRD) and nanoscopic scale observations (AFM, SEM, HRTEM) all confirmed the success of the DA reaction. In addition, thermogravimetric analysis (TGA) revealed that the grafting ratios at ambient temperature and at 80 degrees C were 16.7% and 21.8%, respectively